Charles Mason and the Transit of Venus

It had been a busy day of meetings for me. Each time that I returned to my office, I noticed the same number as a missed call on my telephone. Although I tried calling back, I didn’t realize the person who was so eager to speak to me was in England. Around 10:00 in the evening (his time), Jonathan Peacock of Durham County, in the northeast of England, finally reached me by phone. And did he have an interesting tale to tell.

In his retirement Jonathan Peacock has been researching Jeremiah Dixon, who was born in Cockfield, County Durham in 1733. In 1760 Dixon was commissioned by the Royal Society to go with Charles Mason (then working as a junior assistant at the Royal Observatory at Greenwich), to Bencoolen in Sumatra to observe the transit of Venus on 6 June 1761. Due to contrary winds and a sea battle with the French Navy, their departure was so delayed that they ran out of time and only got as far as Cape Town, South Africa. There they built an observatory and made a very successful set of observations. Afterwards they went back to St. Helena, an island in the Atlantic Ocean off the coast of Africa, where Nevil Maskelyne (later to be 5th Astronomer Royal) had come to make his observations (unsuccessfully due to the weather) on the East Indiaman Mercury (sometimes known as Mercury Snow). Dixon was then sent back to Cape Town with Maskelyne’s clock (made by John Shelton) to check the force of gravity; Mason stayed in St. Helena assisting Maskelyne. They then both returned to England in January 1762, hitching a lift on a passing convoy of East Indiamen. Mason was on the Prince Edward, and Dixon was on the Falmouth.

Mason’s log entry describing the naval battle with a French frigate which forced the ship carrying him to turn back to port.

The transit of Venus refers to the alignment of Earth, Venus, and the sun, such that the orbit of Venus across the sun is visible from Earth. In general, predicting transits of Venus had been complicated, although a pattern related to the months of December and June was finally established for 8, 121, 8, and 105 years before starting over. Astronomers such as Charles Mason (1728-1786) sought to witness and record the timings of this astronomical rarity, because the data allowed them to calculate with greater accuracy the distance between Venus and Earth. As a consequence, astronomers would take that calculation to ascertain the distance between Earth and the sun, as well as the distances from the sun to the other planets.

Jonathan Peacock was calling me because he realized that Ms. Codex 208, held by the Rare Book & Manuscript Library, was undoubtedly the journal kept by Charles Mason on his altered excursion to Sumatra [1]. Mason had been asked to keep a logbook by the Royal Society because of its extreme interest in navigation and accuracy. In the middle of the eighteenth century a ship’s latitude was comparatively easy to determine from the height of the sun at noon. A person didn’t even need to know accurately when 12:00 noon was; he could go on measuring until the sun was at its peak height, which told him when noon was anyway. The problem, however, was longitude. In the Scilly disaster of 1707 four British ships with more than 1,400 sailors were lost in stormy weather; it was later determined that the main cause for the calamity was the navigators’ inability to calculate accurately their locations. The Royal Society looked to the moon, hoping that reliable assessments could be determined by the lunar position. Although the Society was, in fact, producing lunar tables, in 1761/2 they had not yet been published [2]. To build background information, the Royal Society asked Mason to keep his own record, so its members could see how accurate the ship’s master was in his navigation.

-Mason’s log entries describing reaching the Scilly Islands and calculating their position at the end of his return journey in April 1762.

This fascinating piece of history now clarifies not only the purpose behind Penn’s anonymous logbook but also its author. It is definitely written by Charles Mason; in fact, it contains his autograph on folio 87v. It is clearly in his hand, as compared with various pages of his journal made in 1763-8 while surveying in America, (the original is in the National Archives). It is also stylistically similar, for example, in the use of the “astronomical characters” to indicate the days of the week. Finally, he was the only person to travel on that sequence of three ships.

Mason’s signature at the rear of the logbook

In the year following Charles Mason’s trip to capture the transit of Venus, he and Jeremiah Dixon were commissioned to settle a border dispute among British colonies in North America. Their four-year effort resulted in a line of demarcation that formed part of the borders of Pennsylvania, Maryland, Delaware, and Virginia (now West Virginia). The demarcation is known as the Mason–Dixon Line. Through a grant from the National Endowment for the Humanities, a complete digital facsimile, with multiple magnifications, has been made for Charles Mason’s logbook, Ms. Codex 208. It can be viewed at Penn in Hand. Readers everywhere can now study Mason’s notes and calculations. As a start, my colleague Mitch Fraas will be posting shortly about using the logbook to map Mason’s voyages based on the entries for those days during which Mason calculated both longitude and latitude on board ship, a preview of which is shown below.

[2]
The lunar tables weren’t published until 1767. See letter from Maskelyne dated 24 February 1767: “Messrs. Mason and Dixon, Herewith I send you, agreeable to your desire, the nautical almanac of 1767: also a table for facilitating the computation of the moon’s distance from the sun…” Letter bound into NARA RG59 (302029) Journal of Charles Mason, Kept During the Survey of the Mason and Dixon Line, 1763-1768.

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7 thoughts on “Charles Mason and the Transit of Venus”

Jonathon Peacock alerted me to the MS and your excellent blog. I’m plowing thru the MS and have most of it worked out save for the 1/7th of a knot (F) which has me scratching my head. Any clues would be appreciated!
I’m aware he kept the journal as part of the Lunar Distance proving trials and concur its probably part of the missing Ewing papers – it gives hope that more will be discovered!
Best regards,
Edwin Danson
Author, Drawing the Line: How Mason and Dixon Surveyed the Most Famous Border in America (Wiley, 2000)

Thanks for the reply.
Re the 1/7th knot, in 3rd column labelled K and F, the F never exceeds 6 before the K increases, i.e. there are 7x F = 1 K. If K is knot then what is an F (not a fathom and not a foot)? Later in Mason’s calcs he notes the difference between the log lines and adjusts accordingly. Still no clue. I’m working on it!
Re missing papers, I was referring to the papers Mason gave Franklin on his death which could have ended up with Rev Ewing.

I think based on other log books of the time and the instructional literature that the columns are “H” “K” and “F” with H being the hour of the day, K for knots, and F for furlongs. A later manual gives this explanation “F stands for furlongs, a furlong of distance in a log book represents 1/8 of a knot. A half mile would be marked 4, a quarter mile would be marked 2. The furlongs are all added, divided by 8 and added to the knots.” I hope this helps! Also if you are interested in the long-lat data from my map of the voyage I can send you the spreadsheet.
-Mitch

Thanks Mitch. I finally cracked the K and F K=knots and F=fathoms thanks to Moore’s Practical Navigator 1795 and some help from a navy friend who knows about these things. Looking through the three ships logs, Seahorse and Prince Edward use the 7 ‘fathoms’ to a knot convention, Mercury used 6 to the knot. As you point out, there were also mariners using 8 to the knot which I believe was American practice. The fathoms were not our common 6 foot model – Moore talks about 5 foot fathoms, 7 and 7.5 foot version – all supposedly 1/1000th a nautical mile, but as the nautical mile varied so much fathoms likewise varied. Thanks for offer of your spread sheet – answer is yes please! Very kind. It seems mariners were way behind the geodetic times and Norwood’s mile of 1637 still ruled the waves! Wonderful stuff.

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Welcome to Unique at Penn, part of the family of University of Pennsylvania Libraries blogs. Every week this space will feature descriptions and contextualization of items from the collections of the University of Pennsylvania Libraries. The site focuses on those materials held by Penn which are in some sense “unique” - drawn from both our special and circulating collections, whether a one-of-a-kind medieval manuscript or a twentieth-century popular novel with generations of student notes penciled inside. See the About page for more on the blog and to contact the editor.

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